Consider CSTRs (continuous strirred-tank reactors) subject to a tracer impulse experiment. The residence time distribution (RTD) can be found exactly by solving a system of ODEs obtained from mass balances in all reactors or by Laplace inversion of the system's transfer function. It turns out that the first four moments can be easily obtained from the transfer function expression and by means of a Gram–Charlier series for the approximatie RTD. This Demonstration presents a comparison of these three methods for user-set values of , the number of CSTRs in series. It is clear that, as increases, the exact result and the approximate result (derived from the moment expressions in Gram–Charlier theory) show close agreement.